The purpose of this project is (1) to investigate age-related changes in thermoregulation, and (2) to examine the physiological mechanisms underlying these changes. We have demonstrated that aged mice have diminished cold tolerance and are not able to adapt to repeated cold exposure. The cause of these age-related aberrations in thermoregulation appears to be, in part, a reduction in metabolic heat production due to change in brown adipose issue (BAT) and, in part, a reduction in heat conservation. Efferent sympathetic nervous responses to BAT are enhanced in both cold-acclimated and aged animals, which suggests that the sympathetic nervous system plays a major role in cold acclimation, but is not responsible for the aged-related decline in thermoregulation. Our results show that sympathetic nervous activity increases with age to adapt to age-related declines in effector organ function. We have shown the existence of diurnal differences in cold tolerance and metabolic heat production of adult and aged mice: in the afternoon the cold tolerance is poorer and metabolic heat production during cold exposure is lower. One hour of mild exercise prior to cold exposure attenuated diurnal differences in cold tolerance but not in metabolic heat production.